[dgt20 (OP)]

What is the chemical equation when SO2 is added to wine during fermentation?

[Bored chemist]

There are many reactions that take place, so there is no single equation.
One of the most important reactions will be
SO₂ +H₂O --> H₂SO₃
followed by H₂SO₃ --> HSO₃^-  + H⁺

[dgt20 (OP)]

There are many reactions that take place, so there is no single equation.
One of the most important reactions will be
SO₂ +H₂O --> H₂SO₃
followed by H₂SO₃ --> HSO₃^-  + H⁺

What states are they in?

[Bored chemist]

All in solution.

[Bill S]

All in solution.

Sulfurous acid (also sulphurous acid) is the chemical compound with the formula H2SO3. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase.[1] The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogen sulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.

How does a "hitch-hiker" equate these two?

[Bored chemist]

What I wrote as two stages can be considered to be just one reaction
SO₂ + H₂O ⇌ HSO⁻₃ + H⁺     

[Bill S]

So there would not (necessarily) be an intermediate stage in which exsolution of H₂SO₃ would occur?